The present invention relates to an image-forming element and, more particularly, to a photosensitive image-forming element which is capable of providing a graphic arts film of high density and high gradation which is used in the manufacture of a printing plate.
A conventional image-forming element is known wherein a transparent base film, an aluminum-deposited layer and a photosensitive layer are sequentially laminated. Such a conventional image-forming element is described, for example, in Japanese Patent Disclosures No. 51-135,641 and No. 52-99,810. Two major methods are known for forming a graphic arts film with an image using such an image-forming element. In one of these methods, the developing step and the etching step are sequentially performed. In the other method, these steps are performed simultaneously.
According to the former method, a photosensitive layer consisting of a photosensitive material such as polyvinyl cinnamate is developed with xylene which will not etch aluminum. Thus, an etching-resistant resist image is formed on the deposited aluminum layer. Subsequently, that portion of the aluminum layer which is not covered with the resist image is etched with an etchant such as an acidic or alkali etching liquid. An aluminum image is thus formed. The aluminum image obtained by this method has satisfactory light-shielding properties with regard to ultraviolet and visible light rays. Meanwhile, part of the base film such as a polyester film which is not covered with the aluminum image transmits such light rays well.
However, this method requires the use of two different types of processing agents, frequently causing problems. For example, when the resist film portion at the non-image portion is not completely removed in the developing step, etching of the aluminum layer is not sufficiently performed and an irregular aluminum image is formed in the etching step. The method also requires a long treatment time.
The disadvantages of the former method are solved by the latter method since it uses a developer which also serves as an etchant. However, with this method, the resist image must have particularly good resist properties. With a known photosensitive material, the photopolymerized portion of the resist image swells during etching to cause blistering or separation of the resist image from the deposited aluminum layer. Especially in the case of a fine resist image, the resist cannot serve its original purpose, and etching progresses to that portion of the aluminum layer covered with the resist image. In some cases, the aluminum layer may be completely removed. This method can only provide a low resolution and low .gamma.-property, and pinholes are easily formed in the portion of the aluminum image corresponding to a solid region of a wide area.
In order to provide an improvement over these two methods, it has been proposed, as per Japanese Patent Disclosure No. 50-139,720, to deposit on the surface of the deposited aluminum layer a metal which has good adhesion strength with the resist film and which has a smaller ionization tendency than aluminum, thus promoting the etching of the aluminum layer. According to this method, it is possible to improve somewhat adhesion strength between the aluminum layer and the resist image. However, this method still fails to provide good adhesion strength between the aluminum layer and the resist image. It is, furthermore, considerably difficult to deposit another metal on the surface of an aluminum film having a thickness of about 500 to 1,000 .ANG.. Deposition of the metal is time-consuming. Moreover, scratched traces may be easily formed on the metal layer deposited on the aluminum layer correspondent to increase in number of steps, providing only an unsatisfactory graphic arts film.